1. Field of the Invention
This invention relates to a sheet metal end closure for a can or other container. In particular, the present invention relates to a metal end closure having increased resistance to buckling due to internal pressures and to a method and apparatus for forming such an end closure.
2. Description of the Prior Art
The prior art teaches various method for increasing the pressure resistant capability of containers. Increasing the pressure resistance of containers is desirable for various reasons. First, containers having end shells of equivalent gauge can be made to withstand higher internal pressures. And, also, containers having end shells of thinner gauge can be made to withstand equivalent or higher pressures. Being able to reduce the gauge of the material used as end shells for containers, while maintaining adequate internal pressure resistance, has the significant advantage of material cost reduction.
Numerous United States and foreign patents have been issued which disclose various methods and apparatus for forming pressure resistant end shells. U.S. Pat. No. 4,031,837 teaches a method of reforming a conventional end shell by moving a drawing tool into a conventional annular groove while supporting the central wall of the end shell to draw the metal in the process of increasing the depth of the annular groove. U.S. Pat. No. 4,109,599 also teaches a method of forming a pressure resistant end shell for a container in which the reinforcing channel around the end shell is free formed without drawing of the metal. Thus, the thickness of the end shell is not reduced in a final forming operation.
It has been well known for many years to restrike or coin an annular zone or bead around a sheet metal end closure to improve its strength as is disclosed by U.S. Pat. No. 3,441,170. That patent discloses the utilization of loose metal in the end by mechanical doming of the end panel. Other patents that disclose coining of an annular zone or zones around the periphery of the central panel of an end shell to increase the pressure resistance include U.S. Pat. Nos. 4,577,774; 4,641,761; and 4,832,223, among others.
It is also known, as disclosed in published European Patent Application 0103074, to form a metal end shell of increased strength in which the countersink portion is connected to the central panel through at least three reversing curved portions. The published application discloses a coined horizontal portion between two of the reversing curved segments in the end shell. U.S. Pat. No. 3,774,801 discloses a reinforced metal can end having at least two peripheral, radially separated, concave areas of curvature which provide the end with increased resistance against pressure.
It is also known to provide a raised bead or beads in can ends to utilize excess metal that may result from scoring or coining portions of the can ends as disclosed by U.S. Pat. No. 3,554,400. Other disclosures of raised beads include U.S. Pat. Nos. 3,417,898; 4,093,102; 3,326,405; and 3,638,825.
Despite significant progress in the area of increasing the pressure resistant capabilities of end shells for containers, there is still a need for further improvement.
Accordingly, a new and improved pressure resistant end shell and a method and apparatus for forming the same are desired to increase the pressure hold capabilities.